Beta-catenin is an adherens junction protein. Adherens junctions (AJs; also called the zonula adherens) are critical for the establishment and maintenance of epithelial layers, such as those lining organ surfaces. AJs mediate adhesion between cells, communicate a signal that neighboring cells are present, and anchor the actin cytoskeleton. In serving these roles, AJs regulate normal cell growth and behavior. At several stages of embryogenesis, wound healing, and tumor cell metastasis, cells form and leave epithelia. This process, which involves the disruption and reestablishment of epithelial cell-cell contacts, may be regulated by the disassembly and assembly of AJs. AJs may also function in the transmission of the ‘contact inhibition’ signal, which instructs cells to stop dividing once an epithelial sheet is complete.
The AJ is a multiprotein complex assembled around calcium-regulated cell adhesion molecules called cadherins (Peifer, M.(1993) Science 262: 1667-1668). Cadherins are transmembrane proteins: the extracellular domain mediates homotypic adhesion with cadherins on neighboring cells, and the intracellular domain interacts with cytoplasmic proteins that transmit the adhesion signal and anchor the AJ to the actin cytoskeleton. These cytoplasmic proteins include the alpha-, beta-, and gamma-catenins. The beta-catenin protein shares 70% amino acid identity with both plakoglobin, which is found in desmosomes (another type of intracellular junction), and the product of the Drosophila segment polarity gene ‘armadillo’. Armadillo is part of a multiprotein AJ complex in Drosophila that also includes some homologs of alpha-catenin and cadherin, and genetic studies indicate that it is required for cell adhesion and cytoskeletal integrity.
Beta-catenin, in addition to its role as a cell adhesion component, also functions as a transcriptional co-activator in the Wnt signaling pathway through its interactions with the family of Tcf and Lef transcription factors (for a review see Polakis, (1999) Current Opinion in Genetics & Development, 9:15-21 and Gat U., et al., (1998) Cell 95:605-614).
The APC gene, which is mutant in adenomatous polyposis of the colon, is a negative regulator of beta-catenin signaling (Korinek, V. et al., (1997) Science 275: 1784-1787; Morin, P. J., et al., (1997) Science 275: 1787-1790). The APC protein normally binds to beta-catenin and, in combination with other proteins (including glycogen synthase kinase-3b and axin, is required for the efficient degradation of b-catenin. The regulation of beta-catenin is critical to the tumor suppressive effect of APC and that this regulation can be circumvented by mutations in either APC or beta-catenin.
While mammals contain only a single beta-catenin gene, C. elegans contains three (Korswagen H C, et al., (2000) Nature 406:527-32). Each worm beta-catenin appears to carry out unique functions (Korswagen H C, et al., (2000) Nature 406:527-32, Nartarajan L et al. (2001) Genetics 159: 159-72). Because of the divergence of function in C. elegans, it is possible to specifically study beta-catenin role in cell adhesion, which is mediated by the C. elegans beta-catenin HMP-2.
Eukaryotic cells respond to extracellular stimuli by recruiting signal transduction pathways, many of which employ protein Ser/Thr kinases of the ERK family (Levin, D. E., and Errede, B. (1995) Curr. Opin. Cell Biol. 7, 197-202). The ubiquity of ERKs and their upstream activators, the MEKs, in signal transduction was first appreciated from studies of yeast (Herskowitz, I. (1995) Cell 80, 187-197). Part of the cellular response to toxins, physical stresses and inflammatory cytokines occurs by signalling via the stress-activated protein kinase (SAPK) and p38 reactivating kinase pathways (Kyriakis, J. M., and Avruch, J. (1990) J. Biol. Chem. 265, 17355-17363; Kyriakis, J. M., et al., (1991) J. Biol. Chem. 266, 10043-10046; Pulverer, B. J., et al., (1991) Nature 353, 670-674). These stress-responsive kinase pathways are structurally similar, but functionally distinct, from the archetypal mitogen-activated protein kinases (MAPKs or ERKs). The stimuli that start the pathway result in modification of cellular gene expression, growth arrest, apoptosis, or activation of immune and reticuloendothelial cells. TAO1 (thousand and one amino acid) is a protein kinase that may play a role in regulating stress-responsive MAP kinase pathways (Hutchison, M., et al (1998). J Biol Chem 273:28625-32). KIAA1361 is a protein with strong similarity with TAO 1. JIK (JNK-SAPK inhibitory kinase) is an STE20-like serine/threonine kinase and member of the GCK-like subfamily of Ste20 kinases. JIK is activated by ligand-bound EGF receptors, inhibits the JNK/SAPK signaling pathway, and also interacts with IRE1 (a gene involved in endoplasmic reticulum stress response) (Tassi, E., et al (1999) J Biol Chem 274:33287-95; Zhang, W., et al (2000) Biochem Biophys Res Commun 274:872-9; Yoneda, T., et al (2001) J Biol Chem 276:13935-40).
The ability to manipulate the genomes of model organisms such as C. elegans provides a powerful means to analyze biochemical processes that, due to significant evolutionary conservation, have direct relevance to more complex vertebrate organisms. Due to a high level of gene and pathway conservation, the strong similarity of cellular processes, and the functional conservation of genes between these model organisms and mammals, identification of the involvement of novel genes in particular pathways and their functions in such model organisms can directly contribute to the understanding of the correlative pathways and methods of modulating them in mammals (see, for example, Dulubova I, et al, J Neurochem 2001 April; 77(1):229-38; Cai T, et al., Diabetologia 2001 January; 44(1):81-8; Pasquinelli A E, et al., Nature. Nov. 2, 2000; 408(6808):37-8; Ivanov I P, et al., EMBO J Apr. 17, 2000; 19(8):1907-17; Vajo Z et al., Mamm Genome 1999 October; 10(10): 1000-4). For example, a genetic screen can be carried out in an invertebrate model organism having underexpression (e.g. knockout) or overexpression of a gene (referred to as a “genetic entry point”) that yields a visible phenotype. Additional genes are mutated in a random or targeted manner. When a gene mutation changes the original phenotype caused by the mutation in the genetic entry point, the gene is identified as a “modifier” involved in the same or overlapping pathway as the genetic entry point. When the genetic entry point is an ortholog of a human gene implicated in a disease pathway, such as beta-catenin, modifier genes can be identified that may be attractive candidate targets for novel therapeutics.
All references cited herein, including patents, patent applications, publications, and sequence information in referenced Genbank identifier numbers, are incorporated herein in their entireties.